mk: repeato + concato

repeato: produces (or recognizes) a list of n copies of a value, with n
Peano-encoded. Runs forward, backward (recover the count from a uniform
list), and bidirectionally.

concato: fold-appendo over a list-of-lists. (concato (list (list 1 2)
(list) (list 3 4 5)) q) -> ((1 2 3 4 5)).

10 new tests, 378/378 cumulative.

lib/minikanren/relations.sx

@@ -188,3 +188,19 @@
     (conde
       ((== n :z) (== suffix l))
       ((fresh (n-1 a d) (== n (list :s n-1)) (conso a d l) (dropo n-1 d suffix))))))
+
+(define
+  repeato
+  (fn
+    (x n result)
+    (conde
+      ((== n :z) (== result (list)))
+      ((fresh (n-1 r-rest) (== n (list :s n-1)) (conso x r-rest result) (repeato x n-1 r-rest))))))
+
+(define
+  concato
+  (fn
+    (lists result)
+    (conde
+      ((nullo lists) (nullo result))
+      ((fresh (h t r-rest) (conso h t lists) (appendo h r-rest result) (concato t r-rest))))))